pretty: fix ref filtering for %(decorate) formats

Mark pretty formats containing "%(decorate" as requiring decoration in
userformat_find_requirements(), same as "%d" and "%D".

Without this, cmd_log_init_finish() didn't invoke load_ref_decorations()
with the decoration_filter it puts together, and hence filtering options
such as --decorate-refs were quietly ignored.

Amend one of the %(decorate) checks in t4205-log-pretty-formats.sh to
test this.

Signed-off-by: Andy Koppe <andy.koppe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -1961,6 +1961,10 @@ void userformat_find_requirements(const char *fmt, struct userformat_want *w)
 		case 'D':
 			w->decorate = 1;
 			break;
+		case '(':
+			if (starts_with(fmt + 1, "decorate"))
+				w->decorate = 1;
+			break;
 		}
 	}
 }